Objects at any non-zero temperature radiate electromagnetic energy which can be described either as electromagnetic waves or photons, according to the laws known as Planck's law of radiation, the Stefan-Boltzmann Law, and Wien's displacement law. Wien's displacement law states that the wavelength at which an object radiates the most (λmax) is inversely proportional to the temperature of the object as approximated by the following equation:
            λ      max        ⁡          (      μm      )        ≈      3000          T      ⁡              (        K        )            
Hence for objects having a temperature close to room temperature, most of the emitted electromagnetic radiation lies within in the infrared region. Due to the presence of CO2, H2O, and other gasses and materials, the earth's atmosphere absorbs electromagnetic radiation having particular wavelengths. Measurements have shown, however, that there are “atmospheric windows” where such absorption is minimal. An example of such a “window” is the 8 μm-12 μm wavelength range. Another window occurs at the wavelength range of 3 μm-5 μm. Typically, objects having a temperature close to room temperature emit radiation close to 10 μm in wavelength. Therefore, electromagnetic radiation emitted by objects close to room temperature is only minimally absorbed by the earth's atmosphere. Accordingly, detection of the presence of objects which are either warmer or cooler than ambient room temperature is readily accomplished by using a detector capable of measuring electromagnetic radiation emitted by such objects.
One commonly used application of electromagnetic radiation detectors is for automatically energizing garage door lights when a person or car approaches. Another application is thermal imaging. In thermal imaging, which may be used in night-vision systems for driver assistance, the electromagnetic radiation coming from a scene is focused onto an array of detectors. Thermal imaging is distinct from techniques which use photomultipliers to amplify any amount of existing faint visible light, or which use near infrared (˜1 μm wavelength) illumination and near-infrared cameras.
Two types of electromagnetic radiation detectors are “photon detectors” and “thermal detectors”. Photon detectors detect incident photons by using the energy of said photons to excite charge carriers in a material. The excitation of the material is then detected electronically. Thermal detectors also detect photons. Thermal detectors, however, use the energy of said photons to increase the temperature of a component. By measuring the change in temperature, the intensity of the photons producing the change in temperature can be determined.
In thermal detectors, the temperature change caused by incoming photons can be measured using temperature-dependent resistors (thermistors), the pyroelectric effect, the thermoelectric effect, gas expansion, and other approaches. One advantage of thermal detectors, particularly for long wavelength infrared detection, is that, unlike photon detectors, thermal detectors do not require cryogenic cooling in order to realize an acceptable level of performance.
One type of thermal sensor is known as a “bolometer.” Even though the etymology of the word “Bolometer” covers any device used to measure radiation, bolometers are generally understood to be to thermal detectors which rely on a thermistor to detect radiation in the long wavelength infrared window (8 μm-12 μm) or mid-wavelength infrared window (3 μm-5 μm).
Because bolometers must first absorb incident electromagnetic radiation to induce a change in temperature, the efficiency of the absorber in a bolometer relates to the sensitivity and accuracy of the bolometer. Ideally, absorption as close to 100% of incident electromagnetic radiation is desired. In theory, a metal film having a sheet resistance (in Ohms per square meter) equal to the characteristic impedance of free space, laying over a dielectric or vacuum gap of optical thickness d will have an absorption coefficient of 100% for electromagnetic radiation of wavelength 4d. The following equation shows the expression of the characteristic impedance (Y) of free space:
  Y  =                    μ        0                    ɛ        0            wherein ε0 is the vacuum permittivity and μ0 is the vacuum permeability.
The numerical value of the characteristic impedance of free space is close to 377 Ohm. The optical height of the gap is defined as “n×d”, where n is the index of refraction of the dielectric, air or vacuum in the gap.
In the past, micro-electromechanical systems (MEMS) have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. One such application is as a bolometer. Known bolometers use a supporting material which serves as an absorber and as a mechanical support. Typically, the support material is silicon nitride. A thermally sensitive film is formed on the absorber to be used as a thermistor. The absorber structure with the attached thermistor is anchored to a substrate through suspension legs having high thermal resistance in order for the incident electromagnetic radiation to produce a large increase of temperature on the sensor.
A temperature change of an absorber of a bolometer due to absorption of incident radiation is associated with a change in resistance of a thermistors material of the absorber. By measuring an output voltage resulting from applying a probe current across the absorber, the change in resistance in the absorber is determined. Using the correspondence between the change in resistance and the change in temperature of the absorber, the change in resistance of the absorber is used to make an inference about the incident radiation.
The output voltage is a combination of signals corresponding to the temperature change as well as offset, drift, and noise components. The noise component can include flicker (“1/f”) noise and thermal noise. FIGS. 1A-1C illustrates a conventional method of compensating for signal noise components. FIG. 1A is a graph illustrates that a signal 12 corresponding to an output voltage resulting from a probe current comprises noise components including 1/f noise 14 and thermal noise 16. Generally, such noise is compensated for by modulating the signal 12 to a higher frequency band, subjecting the modulated signal to a high-pass filter 18, as illustrated in the graph of FIG. 1B, and by returning the signal 20 back to a base band through demodulation as illustrated in the graph of FIG. 1C. As illustrated in FIGS. 1A-C, a signal-to-noise ratio (“SNR”) increases as a result of the modulation and demodulation scheme.
However if noise, such as flicker noise or low frequency drift noise, is embedded in the bolometer, conventional modulation is insufficient to compensate for the noise components. What is needed, therefore, is a method of modulating a bolometer that can compensate for a noise source embedded in the bolometer.